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Radiation Risk of Medical Imaging for Adults and Children

Authors: Mr Anthony Wallace* Dr Timothy Cain *

What is radiation – am I exposed to background radiation each day even if I do not have an X-ray examination?

Background radiation

Background radiation refers to the ionising radiation from high energy particles or rays that we are unavoidably exposed to in our daily lives, which gives each of us a small but continuous dose of ionising radiation.

Part of background radiation is due to the electromagnetic radiation spectrum, and this includes ‘ionising’ components such as X-radiation (X-rays) and gamma rays, and ‘non-ionising’ components such as visible light and radio waves.

X-rays, gamma rays and some other high energy particles are called ‘ionising radiation’ because they can deposit enough energy into a body tissue to change its molecules or proteins by ejecting an electron from an atom.

The sources of ionising radiation in our environment are cosmic rays from the universe, naturally occurring radioactive substances in the food and water we eat and drink, the air we breathe, in the ground, in building materials, and so on.

We are all weakly radioactive due to the presence of radioactive elements in our bodies (such as potassium 40 and carbon 14), and this contributes to our background radiation exposure.

Background radiation is most commonly given in units of millisievert (mSv), which both measures and combines the radiation dose and the consequent risk delivered by an exposure.

The amount of background radiation varies widely in different parts of the world due to the radioactivity of the soil, latitude, height above sea level and lifestyle (predominantly indoors or outdoors). In Australia, the background radiation is estimated to be 2 mSv, which is approximately equivalent to 100 single chest X-rays per year. A few parts of the world have background radiation 10 or more times greater than that generally found in Australia, but there are no studies that have shown an increased risk of cancer in populations living in areas with a higher background radiation level.

X-rays

Visible light and X-rays both travel in straight lines, and cast a shadow when they interact with a solid object.

X-rays have more energy than visible light, and can go much deeper into and through objects. An

X-ray beam is absorbed differently by different parts of the body, and these differences make shadows that are used to create an image or picture.

A dense structure, such as bone, absorbs a high percentage of an X-ray beam (appears light grey on the image); whereas low-density structures, such as soft tissues (e.g. muscle and skin), absorb a small percentage (appears dark grey). Metal objects will usually show up as white and air will usually appear black.

Man-made X-rays are electrically generated and are only present when the X-ray machine is switched on, just like a light bulb.

Once the X-ray machine is switched off, there is no radiation coming from the X-ray machine. Computed tomography (CT) is a specialised X-ray examination using powerful computers to make the pictures. Having an X-ray or CT examination does not make a patient radioactive.

X-ray procedures provide a measurable additional radiation dose to our annual background radiation exposure. The contribution of medical X-rays to the average population radiation dose has increased over the past few decades, as more sophisticated imaging tests have become available; this is especially true of CT scans.

Which kinds of tests are associated with Ionising radiation and which ones are not?

Radiology is the medical specialty that uses X-rays, ultrasound and magnetic resonance imaging (MRI) to produce images or pictures that help to diagnose disease or injury, or guide procedures that treat the disease or condition. Ultrasound and MRI do not use or produce ionising radiation.
Nuclear medicine is the medical specialty that uses radiopharmaceuticals (radioactive medicines) to show how an organ is working. Certain radiopharmaceuticals can also be used to treat some medical conditions, such as some cancers.

These medical specialties are often carried out in the same radiology facilities and are jointly called ‘medical imaging’.

There are generally four types of technology used to carry out different types of medical imaging procedures:

Imaging using an X-ray tubeX-rays are a type of ionising radiation produced by an X-ray tube, which is found in equipment used for the following types of imaging procedures:
Computed Tomography (CT)
Fluoroscopy (where the images produced are displayed like a movie on a television screen)
Plain radiology/X-ray film, digital and computed radiography (see Plain Radiography/X-rays)
Mammography (see Diagnostic Mammography)
The radiation exposure from having an X-ray, fluoroscopy, mammography or CT examination only occurs while the machine is on and activated by the operator. Any operator of X-ray machines has to be trained in the use of these machines and holds a government licence.

Magnetic resonance imaging (MRI)
MRI uses strong magnetic fields and radio waves to make images. It does not use ionising radiation (see Magnetic Resonance Imaging (MRI)).

Ultrasound
Ultrasound uses high frequency sound waves that the human ear cannot detect to make images. It does not use ionising radiation (see Ultrasound).

Nuclear Medicine
Nuclear medicine uses small amounts of a radioactive material (radiopharmaceutical), which is normally injected into a patient’s vein, but can be given through the mouth or inhaled. The patient becomes slightly radioactive for a short time, and images are made from the radiation given off from the patient. Positron emission tomography (PET) is a special kind of nuclear medicine study that uses short-lived radiopharmaceuticals that provide specialised information, particularly for investigation of cancers and epilepsy (see Nuclear Medicine and PET scan).

Which tests give me the most ionising radiation and which ones the least?

The radiation exposure a patient receives will depend on the type of examination and the purpose of the imaging study. Generally, plain X-rays, mammography and fluoroscopy give a lower radiation dose than CT, but complex procedures using fluoroscopy can result in doses similar to extensive CT examinations. Ultrasound and MRI do not use ionising radiation. Because children are more sensitive to the effects of ionising radiation it is important, where possible, to use tests that do not require ionising radiation (i.e. ultrasound and MRI) providing they will give the answers required by the referring doctor.

What are the risks associated with radiation from diagnostic X-ray imaging and nuclear medicine procedures?

These risks are difficult to accurately measure, but it has been shown that the risk of developing cancer is slightly increased if you have been exposed to additional ionising radiation above background levels. The risks are not the same for all people; females are slightly more sensitive to the effects of ionising radiation compared with males. Children are also more sensitive, as the cells that make up their growing tissues and organs are dividing more rapidly. Children also live longer, so the effects of radiation have more time to become visible.

Some people have genetic differences that predispose them to the effects of ionising radiation. There are other risks from high exposures to ionising radiation, but these are not expected at the dose levels used in diagnostic imaging.

The increased risk is small, and usually less than the risk from not identifying or treating a disease or condition properly. It is important to make sure that every test has a definite benefit to balance the small radiation risk of the test. If you are referred for a CT scan or other test involving ionising radiation, it is important that you discuss the relative risks and benefits with your referring doctor so that you understand how you will benefit from having the study.

All operators of an X-ray machine (including CT) have been trained to use only enough X-rays to provide quality pictures for the specialist. The dose of ionising radiation is therefore kept to a minimum.

What are the benefits of diagnostic radiology using ionising radiation?

The purpose of diagnostic radiology is to provide the radiologist or nuclear medicine specialist (specialist doctors) with images of sufficiently high quality, so that they can report the results of the test to your doctor to assist in understanding and explaining your medical problem or symptom, and confirm either the presence or absence of disease or injury.

X-ray imaging procedures, in the main, offer the advantage of being:

low risk;

non-invasive;

fast;

accurate;

well established as an investigative technique.

It is important that any request for an imaging test is provided by your doctor, in consultation with you. It is your own doctor who will be able to make an assessment of whether the benefits of the X-ray procedure outweigh any possible risks. The radiologist or nuclear medicine specialist supervising the procedure will also assess if it is the most appropriate test, taking into account the information your doctor has written on the request form together with your medical history. If there are concerns, then the radiologist/nuclear medicine specialist might want to speak to your doctor before the test is carried out.

How do I decide whether the risks are outweighed by the benefits of exposure to X-radiation when I have a radiology test or procedure?

Your decision should be made in close consultation with your referring doctor. Ask your doctor about the procedure and how it will help to provide information about your symptoms or the presence of disease or injury. Ask your doctor about the risks of the procedure and what the risks would be of not having the procedure; that is, if your doctor needs the information in order to identify and plan the most appropriate treatment.

Although there is a small risk of harm from ionising radiation, there could be a greater risk of not having the information; for example, failure to detect potentially serious disease that could be easily treated at an early stage, but is harder to treat or is incurable if detected later.

Discuss any concerns with your doctor, and access reputable websites to find out information. You might also be able to obtain information from the hospital or private practice where your doctor has referred you for the procedure.

It might also be as beneficial to you to confirm the absence of disease or injury as it is to confirm its presence.

Are there alternatives to procedures that involve ionising radiation that would answer my doctor’s question?

X-rays, CT scans, nuclear medicine studies, MRI and ultrasound each have a greater or lesser ability to scan and provide an image of specific parts of the body and/or to identify the presence or absence of certain conditions or disease. MRI and ultrasound studies are usually used in preference to other imaging tests when it is possible to do so. Your referring doctor will consider which imaging procedure is most appropriate depending on the type of information required and your medical history. Your doctor can also discuss the most appropriate choice of test with the radiologist or nuclear medicine specialist.

Further information

The amount of ionising radiation that a child is exposed to in X-rays is usually altered to take into account their body size; smaller bodies require less X-rays to make the images, as there is less tissue to ‘stop’ the X-rays. It is important that this also happens when a child has a CT scan. Older CT scanners were not designed to alter the radiation dose for small patients, but newer CT scanners have much better detectors (that measure the X-rays passing through the patient), and can significantly reduce the radiation dose to minimise the absorbed dose of ionising radiation. It is important that a modern CT scanner is used for children, and that the operator adjusts the imaging parameters to reduce the radiation dose to an acceptable level.

Useful websites:

Australian Radiation Protection & Nuclear Safety Agency
www.arpansa.gov.au
www.arpansa.gov.au/radiationprotection/factsheets/index.cfm
www.arpansa.gov.au/radiationprotection/Basics/xrays.cfm
Image Gently
www.pedrad.org/associations/5364/ig/
International Atomic Energy Agency
rpop.iaea.org/RPoP/RPoP/Content/index.htm
National Council on Radiation Protection & Measurements
www.ncrponline.org/
Health Direct – CT scans and radiation exposure in children and young people resource page. The information for parents and carers page includes access to videos and interactive games.
www.healthdirect.gov.au/ctscansforkids
www.healthdirect.gov.au/ct-scans-for-parents-and-carers
Health Physics Society
www.hps.org/
Royal Australian and New Zealand College of Radiologists
www.ranzcr.edu.au

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RANZCR® intends by this statement to exclude liability for any such opinions, advices or information. The content of this publication is not intended as a substitute for medical advice. It is designed to support, not replace, the relationship that exists between a patient and his/her doctor. Some of the tests and procedures included in this publication may not be available at all radiology providers.

RANZCR® recommends that any specific questions regarding any procedure be discussed with a person's family doctor or medical specialist. Whilst every effort is made to ensure the accuracy of the information contained in this publication, RANZCR®, its Board, officers and employees assume no responsibility for its content, use, or interpretation. Each person should rely on their own inquires before making decisions that touch their own interests.